Allow taking out raw bytes from a SubmittableExtrinsic

[MOZGIII]

This is useful for something like this:

    let balances_transfer_tx = ...;

    let signed_balances_transfer_tx = api
        .tx()
        .create_signed(&balances_transfer_tx, signer, Default::default())
        .await?;

    let extrinsic_hash = Hashing::hash(signed_balances_transfer_tx.encoded());

    // We need `extrinsic_hash` because we want to log it, before we try to submit and watch the extrinsic.
    // We can't use `signed_balances_transfer_tx.submit_and_watch()` as we want to avoid doing the work twice.
    debug!(
        message = "Balance transfer extrinsic signed",
        ?address,
        ?extrinsic_hash
    );

    // Manually do the `watch_extrinsic`.
    let sub = api
      .rpc()
      .watch_extrinsic(Encoded(signed_balances_transfer_tx.into_encoded())) // <---- this is where we need the new API
      .await?

     let mut progress =
        TxProgress::<chain::HumanodeConfig, _>::new(sub, api.clone(), extrinsic_hash);

    // Continue as if we called `signed_balances_transfer_tx.submit_and_watch()`...